[outofcoffee]

Why

--env implied environment variables, but the domain word has always been node — a fleet node to pin a launch or a drive to, or a registered cloud environment when no fleet is in force. Align the external flag with the internal vocabulary.

Closes #236.

What changes

• The launch's two target flags — the fleet-pin --node and the renamed --env — merge into one -n/--node.
• --fleet names the fleet, --node names the node within it — flagged or discovered, and on every command: the launch pins to it, the read commands narrow to its row, fleet start/stop take it in lieu of the positional name. A name a named file lacks fails naming the file and its nodes; only beside a discovered fleet.yaml does an unknown name fall back to a registered environment, since a file that merely sits in the directory states no intent. (An earlier draft refused the pair on the fleet commands; review caught that the launch path already accepted it, making the refusal the odd half — checkOneTarget is gone.)
• --env is removed outright, no compatibility alias. splitHarnessArgs catches a forwarded --env/-e on the launch and names the new spelling.
• -n is the short form everywhere except fleet deploy and cloud deploy, where -n stays --dry-run and the node is named long-form only.
• Genuinely environmental spellings are untouched: the ENV instruction, .env files, --print-env, --token-env, and the cloud env command (only its target flag becomes --node).
• --node completion on the fleet commands now offers the fleet in force's node names as well as registered environments, honouring an explicit --fleet value on the line.

Notes

• Internal cloud package vocabulary (Environment, the clouds/<name>/cloud.json registry, the AWS env wire param) is deliberately not renamed — this is CLI surface only.
• Specs, design and docs updated under openspec/changes/rename-env-flag-to-node/.
• Verified end-to-end with stub daemons (15 checks): narrowing via discovered and named fleets, the ghost-name refusal wording, pinned launch carrying the node's endpoint with its engine key travelling only through the harness's environment (never to disk), and fleet start -n riding the node's Spinloop as a deploy config.

#236